Such corrugated strip packaging parts of cardboard, pasteboard or the like have been known for quite a while. The technology of molded plastic packaging has largely squeezed corrugated parts of cardboard or pasteboard out of the market in recent years, however. This is explained not only by the very rational production methods for plastic molded parts, but also by the fact that plastic molded parts can easily be specially adapted to the form of any object to be packaged. In particular, the undercuts retaining the objects in the indentations are easier to reach in the molding of plastic than in the corrugation of cardboard.
In general, repackaging and other packaging parts of cardboard or corrugated board, in particular micro-corrugated board, are also used for the objects inserted into molded plastic parts. This results in packaging waste of two different materials which causes sorting and processing problems especially in view of the recent environmental awareness. We are therefore seeing a trend toward the comeback of corrugated strip packaging parts of cardboard or pasteboard, in such a way that the packing consists entirely of fibrous material that can be returned to paper production.
As already mentioned, however, there are problems in processing cardboard and pasteboard into such corrugated strip packaging parts in a precise, appealing shape.
A typical machine for producing corrugated strip packaging parts can be seen in German Patent DE-B-1 015 673, for example. This patent is somewhat confusingly entitled "machine for producing corrugated board". As one can easily see from the drawings of the patent, however, the procedural principle on which the described device is based lies in the fact that a sequence of cylindrical rods at a distance from each other is held at the perimeter of two separate gearwheels, and that the length of material to be molded into the undulation by means of a toothed roller representing a form tool is pressed into the intervals between the rods. In the process, the length of material is placed against the perimeter sections facing the toothed roller to form the lower loops of the undulation. As can easily be seen from FIG. 1, it is not desirable to make the upper loops of the undulation very wide, since on the one hand, an unnecessary amount of space is wasted for this and on the other hand, the undercut effect is not achieved. The ideal shape of the undulation is therefore achieved when the legs of the upper loops more or less touch each other halfway up, but the upper loops somewhat widen above this point of contact and form a well defined radius.
In shaping, there is a limit to how narrow the upper loops can be formed, because otherwise a form tool will not be able to engage. Therefore, in the device according to German Patent DE-B 1 015 673, the freshly formed corrugated strip, which still contains the cylindrical rods as forming bodies in the lower loops, is pushed together on a basically level stretch, during which the upper loops narrow but the lower loops remain unchanged because of the rods in them. In this pushed together condition, the undulation is then glued onto a carrier sheet, in such a way that the forming bodies can subsequently be removed without the undulation changing again.
In pushing together the undulations according to the method illustrated in German Patent DE-B 1 015 673, well formed upper loops, as shown in FIG. 1, are obtained at best only if a very long-fibered, elastic material is used that does not tend to bend or break in the summits of the upper loops. For packaging parts of the type concerned, however, it is desirable to use inexpensive fibrous materials containing either a large portion of wood-pulp or also reclaimed fibers from used paper. These products are generally far less elastic than lengths of fresh, long-fibered cellulose.
During processing in a device according to German Patent DE-B 1 015 673, such a less elastic material does not tend to maintain the desired, continuously curved shape in the area of the upper loops when the undulations are pushed together, but rather it tends to bend or break, in such a way that the undulation looses its curvatures in its upper area and takes on the form of a series of joined together U's. This process is favored by the fact that when the undulation is pushed together according to the known method, the upper loops can slide freely upwards in such a way that during pushing together, there is a flattening of the upper loops while their height and thus their bending increases.